1. Field of the Invention
This invention concerns discharge lamps, and in particular, the manner in which each of the electrodes is connected to the tip of an electrode support bar within a bulb of the lamp.
2. Description of Related Art
Conventionally, a discharge lamp has a bulb that comprises a light-emitting tube, which forms the light emitting space, and seal tubes connected to it on opposite sides. Sealed within the light-emitting tube of the bulb is a pair of facing electrodes (anode and cathode) made of tungsten, xenon gas, for example, and mercury. Each of the electrodes is fixed to the tip of a supporting electrode support bar that is made of tungsten and extends along the tube axis from the seal tube of the bulb into the light-emitting tube.
A well-known means of fixing the electrode to the tip of the electrode support bar, is to form a concavity in the base surface of the electrode, and fit the tip of the electrode support rod into the cavity with a buffer material between them. To explain more concretely, FIG. 7(a) shows a tapered concavity 50a formed in the base surface 50b of the electrode 50, such that the diameter of the concavity grows smaller from the base toward the tip of the electrode 50. The tip 51a of the support bar 51 is also tapered, such that its diameter grows smaller moving toward the tip of the bar, to match the shape of the concavity 50a of the electrode 50. The buffer material 52 is formed around the tip 51a of the electrode support bar 51. Next, as shown in FIG. 7(b), the tip 51a of the electrode support bar 51 is inserted into the concavity 50a of the electrode 50 and, as shown in FIG. 7(c), by pressing the tip 51a of the electrode support bar 51 so that the entire tip is accommodated within the concavity 50a of the electrode 50, the tip 51a of the electrode support bar 51 is fitted into the concavity 50a of the electrode 50, and is fixed to it.
Nevertheless, there are the following problems with such a technique.
(1) The buffer material 52 is formed by wrapping molybdenum foil or a tantalum sheet around the tip 51a of the electrode support bar 51, but adjustment of the thickness of the buffer material 52, i.e., adjustment of the thickness of the molybdenum foil or tantalum sheet used or of the number of turns wrapped, is determined by the fabricator through repeated trial and error, and so this operation is quite complex and requires relatively advanced technology. Moreover, after the tip 51a of the electrode support bar 51 is fitted into the concavity 50a of the electrode 50, a procedure is required to trim away any excess buffer material 52 that is exposed outside the concavity 50a of the electrode 50. In addition, both the concavity 50a of the electrode 50 and the tip 51a of the electrode support bar 51 are tapered to facilitate the insertion of the tip 51a of the electrode support bar 51 into the concavity 50a of the electrode 50, and the machining process to form the tapered concavity 50a in the electrode 50 and that to form the tapered tip 51a of the electrode support bar 51 require a high degree of machining precision. For those reasons, this means of fitting and fixing the tip 51a of the electrode support bar 51 into the concavity 50a of the electrode 50 takes time and effort, and so a high level of productivity is not possible.
(2) The holding power of the electrode support bar 51 on the electrode 50 (the force necessary to remove the tip 51a of the electrode support bar 51 from the concavity 50a of the electrode 50) depends on the thickness of the buffer material 52. Moreover, adjustment of the thickness of the buffer material 52 is done by repeated trial and error by the fabricator, as stated above, and so it is difficult to fix the electrode 50 to the tip 51a of the electrode support bar 51 with a holding power that is consistent from one product to the next. As a result, there is great fluctuation in the holding power of the electrode support bar 51 on the electrode 50 from one product to the next, and if the electrode support bar 51 has too little holding power on the electrode 50, the electrode 50 could fall off the electrode support bar 51 during shipping of the discharge lamp, or when the lamp is lighted.
In view the situation described above, it is a primary object of this invention to provide a discharge lamp in which, by a simple procedure, the tip of the electrode support bar can be fitted easily into a concavity formed in the base of the electrode, and can be stably fixed to the electrode with high holding power.
The discharge lamp of this invention is a discharge lamp having an electrode supported by an electrode support bar, preferably one made of molybdenum, by means of the tip of the electrode support bar engaging in a concavity formed in the electrode, in which the tip of the electrode support bar is made with a slit into which a spreader piece is pressed to spread the slit.
More specifically, in accordance with the invention, a spreader piece is mounted in a slit formed in the tip of the electrode support bar, and when the tip of the electrode support bar is pushed into the concavity in the electrode in that state, the spreader piece is pushed, by the bottom of the concavity in the electrode, into the slit in the electrode support bar. As a result, the slit is spread and the outer surface of the electrode support bar puts pressure on the inner surface of the concavity in the electrode, by which means the electrode is fixed to the tip of the electrode support bar. Thus, there is no need to use buffer material, and there is no need to taper the concavity in the electrode or the tip of the electrode support bar, and so the tip of the electrode support bar can be fitted easily to the concavity in the electrode by a simple process. Moreover, because a slit in the tip of the electrode support bar is spread and the outer surface of the electrode support bar is pressed against the inner surface of the electrode, the electrode is fixed to the tip of the electrode support bar stably and with great holding power. In addition, by forming the electrode support bar of molybdenum, which has high plasticity and toughness, it is possible to prevent damage to the tip of the electrode support bar when the slit is spread.